The 2,4-diaminopyrimidine derivative compounds have been extensively studied as inhibitors of dihydrofolate reductase (DHFR). In the living cell, DHFR reduces dihydrofolate to tetrahydrofolate, which is further used to produce 5,10-methylenetetrahydrofolate, a substance essential for DNA synthesis and cell growth. Inhibition of DHFR results in inhibition of DNA synthesis, and in cell death. Many DHFR inhibitors, also known as antifolates, are therefore potentially useful drugs against infectious agents, provided that they can specifically inhibit DHFR of the target cells without substantially affecting the cells of the host.
Many 2,4-diaminopyrimidine derivative compounds have been synthesized and shown to have antimalarial activities. These compounds include pyrimethamine (Pyr) and derivatives thereof, with substituents on the 5-phenyl group, and substituents on position 6. The synthetic processes generally involve three steps: a) preparation of keto nitrile via an acylation of the arylacetonitrile with ester as catalyzed by an alkali alkoxide, b) preparation of the corresponding β-alkoxyacrylonitrile or the corresponding ketal of the keto nitrile, c) preparation of 2,4-diaminopyrimidine by treatment of the above intermediate with guanidine in alcoholic solution. Due to low yield of keto nitrile together with decomposed/polymerized compounds of the previous methods, the present invention provides a modified method, employing effective acylating reagents, base, reaction conditions and solvents in three steps, for the preparation of pyrimethamine derivative compounds in better overall yields.
Although pyrimethamine and other described 2,4-diaminopyrimidine derivative compounds are effective against wild type malaria parasites, they are not effective against resistant parasites, which have been shown to bear mutations in the dihydrofolate reductase, for examples, mutations of Plasmodium falciparum dihydrofolate reductase at positions 108 (serine to asparagine), 51 (asparagine to isoleucine), 59 (cysteine to arginine) and 164 (isoleucine to leucine). The degree of resistance generally increases with the number of mutations the parasite accumulates, prompting the need for novel drugs which are effective both against sensitive and resistant strains of malaria parasites. These drugs must, moreover, be of low toxicity to the human host. It is also preferable that these drugs do not have significant antibacterial activity, since they often have to be administered over extended periods. Under such circumstances, there would be a danger of development of resistant strains of bacteria if the drugs also have significant antibacterial activity.
The present invention describes compounds which are effective against malaria, in particular drug-resistant malaria arising from mutations as described. A number of these compounds have low toxicity to the human host and are therefore useful in suitable pharmaceutical compositions. They have little antibacterial activities and are unlikely to lead to development of resistant strains of bacteria on deployment. The procedure for their synthesis is also described.